Casting resins are often mixed with fillers, for example powdered quartz, which are very abrasive. Piston seals, which lie closely to the cylinder wall, can in this case be used only to a limited degree, since the jamming of filler particles between the seal and cylinder wall would lead to a quick destruction of the seal and cylinder wall. A sufficient lifetime is obtained only when a gap is left between piston and cylinder wall, the width of which gap is so great that filler particles cannot be jammed therein. However, certain leakage flows are created by such gaps, which leakage flows are particularly disadvantageous when several components of the casting resin must be mixed together in a specific proportional relationship and subsequently guided to a mold and when during the gelling of the casting resin pressure is supposed to be maintained in the mold. During this pressure maintenance, only small amounts of resin per unit of time are further pressed into the mold, so that different leakages in several pump cylinders result in significant recipe (mixing proportion) adulterations.
Known (from German Pat. No. 27 48 982) is a system, in which the casting resin components are fed by means of dosing pumps to a mixer. Several buffer elements are fed from the mixer. A mold is associated with each buffer element. Said arrangement permits the pressure regulation in several molds to be independent from how long the gelling operation in the individual molds lasts. The buffer elements contain pistons, onto the one end of which acts a pressure medium, while the other piston end presses onto the casting resin and in this manner maintains a specific pressure in the associated mold and also displaces casting resin into the mold in order to compensate for the volume loss which takes place during gelling. The system is complicated, since aside from the dosing pumps also buffer elements are needed.
Further known (from German OS No. 25 54 233) is also a system with two pump cylinders with pistons, the piston rods of which project from the cylinders and are connected to a connecting bridge in order to force a synchronous movement of the pistons. This principle of the dosing is also known otherwise in the dosing technique, for example from German OS No. 23 24 098. The piston rod sides of the pump cylinders according to German OS No. 24 54 233 are loaded by a pressure medium.
Casting resin components which are packed into sacks are introduced into the cylinder chambers on the other end of the piston from the piston rod end. During closing of the cylinder chambers the sacks are slitted. The components are guided together in a spray gun. Nothing is said in the reference concerning the fit of the pistons in the cylinder.
The basic purpose of the invention is to construct an injection-molding system of the abovementioned type so that casting resins, including those with abrasive fillers, can be processed without creating leakages of a damaging degree out of the pump chamber.
This purpose is attained according to the invention by the piston being sealed off relative to the cylinder wall by a gap seal, by the casting resin or the casting resin components being introduceable free of a casing into the pump chamber, by the driving fluid being compatible with the casting resin or the casting resin components and by the active surfaces of the piston which come into contact with the driving fluid and the casting resin or the casting resin components being at least approximately of the same size.
The sealing of the piston by a gap seal makes the pump cylinder, or the pump cylinders, insensitive to abrasive fillers which are contained in the casting resin. Filler particles thus can penetrate into the gap between piston wall and cylinder wall without being jammed there. They therefore also cannot lead to the formation of grooves (scoring) which damage the piston and the cylinder wall. Since there is no seal which lies closely against the cylinder wall, destruction of such a seal cannot occur. Still leakages are practically completely avoided, since the same pressure exists on both ends of the piston, so that there exists no pressure gradients (drops) which could result in leakages. When the driving fluid is, as is preferable, supplied on the piston rod end of the piston, the pressure on the driving end of the piston is, because of the piston rod cross section, slightly greater than on the piston end which presses the casting resin into the mold or toward an outlet. Therefore, at most only small fluid amounts of the driving fluid could penetrate into the casting resin, while in this case by no means does casting resin get into the driving fluid. Due to the use of a driving fluid which is compatible with the casting resin, no disadvantages arise from the two fluids being mixed with one another to a small degree in the area of the gap between piston and cylinder.
The driving fluid preferably comprises at least in part the casting resin or a casting resin component. This, however, is not a condition for the realizing of the invention. The driving fluid can be any neutral fluid, which does not have a damaging influence on the casting resin, that is, does not change same in such a manner that the product is impaired. However, it is advantageous that the driving fluid does not contain any abrasive sedimentating fillers. Also the driving fluid should not harden. However, no fluid may be used as driving fluid which effects an undesired reaction in the casting resin or the casting resin component. For example, one will not use as driving fluid a hardener, if the respective cylinder is supposed to convey a hardness-free casting resin component.
The driving fluid can be pressurized in various ways. Particularly simple for this is the use of a pressurized gas.
In a preferred embodiment of the invention, the capacity of the pump chamber is larger than is needed for the complete filling of a single mold. Such an injection-molding system makes possible the complete filling of a mold and a subsequent pressure maintaining in connection with a follow-up pressure on the casting-resin mass, and thus the pressure regulating mentioned in the beginning, whereby the system has an extraordinarily simple design. The stroke size can also be dimensioned such that several individual portions can be produced with one piston stroke, in order to for example inject one charge of small parts in a vacuum chamber. During the break, which is created by ventilation, loading and evacuation of the casting chamber, the pump cylinder can be loaded anew.
A drive mechanism is advantageously associated with the piston. A loading stroke can be produced with such a drive mechanism, by which loading stroke can be sucked into the cylinder the mass which is to be conveyed later. However, a filling of the cylinder is also possible without such a drive mechanism, when the filling for example occurs under the action of the force of gravity or when a loading pump is provided.
By using several pump cylinders, thus in the case of a system in which several casting resin components are supposed to be mixed together according to a specific recipe (formula), a mechanical connection between the individual pistons is provided. This mechanical connection can in one simple design consist of a rigid connecting piece. Such connecting pieces are actually known, for example from the abovementioned German OS No. 25 54 233, but without a separate guide which is parallel with the cylinder axes. Such known piece has the disadvantage that a cocking of the pistons in the cylinder or the piston rods in their guides can be created, and the piston rods must then be constructed relatively thick, which results in greater pressure differences between the two ends of the piston. However, in the invention, by using a separate guide, thin piston rods can be used, which as a rule need absorb only pulling forces (tensile forces), so that the piston front end and the piston back end are approximately of the same size and thus the pressure difference on both ends of the seal gap is approximately zero. In place of a rigid connecting piece it would also be possible to provide for the mechanical connection a lever, which can pivot about a stationary fulcrum point and which operates several cylinders, which cylinders at their other ends are secured on a base. This permits effecting, with equal cylinder diameters, different volumetric deliveries per unit of time. Such demands are actually known, for example from German OS No. 27 12 610. In the case of a rigid connecting piece different cylinder diameters must be used, if the volumetric deliveries per cylinder are supposed to differ. This is possible without any problems in fitting gap seals, since fitting of elastic seals is not needed.
In a particularly simple construction of an inventive system, the mechanical connection is done by the pistons being parts of a differential piston. Separate connecting pieces are not needed in this case even if several cylinders are provided. In such a device it is advantageous to have close spaced annular chambers which surround sections of the differential piston and connect respective pump chambers to the mold or the like, since in the area of the seal gap noncompatible components leaking from one annular chamber to the other are quickly flushed through due to the constant passage of material through the annular chambers to the mold. The annular chambers may also be directly adjacent, possibly combined to one annular chamber.
The inventive injection-molding system is advantageously equipped with evacuatable storage containers for the casting resin. Qualitatively high-grade workpieces, for example electrical structural parts, like spools, must be created so that no cavities (shrink holes) exist in the injected casting mass. This can only be achieved if the casting resin or its components are carefully degassed prior to casting. The processing of degassed components is reliably possible with the inventive principle.
The inventive system is advantageously equipped with a valve construction by which reliable sealing of the inflow after the filling of the pump cylinder is achieved, even if the casting resin contains abrasive particles. Also, this valve operates according to the principle of the pump cylinders, namely the valve closing force is produced by a driving fluid which is compatible with the casting resin. Also in the closed condition of the valve, with a pump cylinder which is under pressure, there does not exist a pressure difference between the driving fluid and the casting resin or the casting resin component.
Several pump cylinders can be connected to a storage container which contains premixed (ready mixed) casting resin. With this, several injection points can be supplied independently from one another from one storage container.
Casting resin can be ejected in small individual portions with an apportioning means so that for example the filling of several small molds with one piston stroke is possible. The apportioning means can be either a passive device, that is a device which is operated only by the driving fluid, or instead an active, pumplike device. A pressure increase is also possible with an active device.